Light sensor with modulated radiant polychromatic source

ABSTRACT

An apparatus is described for assessing plant chlorophyll content remotely sensed by the invention thereby allowing selective monitoring or treatment of individual plants. In one preferred embodiment, a polychromatic emitter provides light beams substantially in the red edge portion of a plant&#39;s reflectance spectrum. This light beam illuminates a surface area on the plant, which may be bare ground or plants. The beam of light may be focused, collimated or non-focused. A detector array, usually composed of an array of spectrally sensitive detectors, detects portions of this polychromatic light beam reflected by the surface area and provides a signal indicative of the change in chlorophyll status by determining the wavelength of the red edge inflection point REIP. In another preferred embodiment of the invention, an array of sequentially pulsed monochromatic emitters provides light beams having wavelengths substantially along the red edge portion of a plant&#39;s reflectance spectrum. These light beams illuminate a surface area on the plant, which may be bare ground or plants. The beams of light may be focused, collimated or non-focused. A photodetector detects the light reflected by the surface area and provides a signal indicative of the change in chlorophyll status by determining the wavelength of the red edge inflection point REIP. In both embodiments, a controller analyzes the resulting REIP wavelength and responds by activating a device to take some action with respect to the plant or stores the analyzed signal with corresponding DGPS position in the controller&#39;s memory for later analysis.

This invention is a continuation in part to U.S. patent application Ser.No. 10/703,256

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a structure and a method for determiningchanges in the chlorophyll status of a plant via remote sensing of theplant's reflectance spectrum spanning from approximately 650 nm to 800nm.

2. Description of Related Art

In order to manage our natural resources in an efficient andcost-effective manner, producers and turf professionals need a way inwhich to measure and assess the health and performance of theirlandscapes. For example, the need to know when and how much fertilizer(nitrogen) and other nutrients to apply to a plant to elicit theappropriate growth response is primarily guess work to the producer.Because nitrogen is required by the plant in the greatest quantities andbecause nitrogen is rather mobile in soils, producers have practiced aone time application of nitrogen to cover the crops need for the entiregrowing season. However, over application of nitrogen on agriculturaland commercial landscapes has resulted in the contamination of groundand surface waters. The primary vectors for water contamination arerun-off and leaching. Nitrate-nitrogen is the most common contaminantfound in U.S. groundwater. Nitrate contamination is increasing both inarea and concentration, particularly beneath landscapes dominated bycorn production. It is estimated that 1.8×10⁹ kilograms of nitrates washinto the Gulf of Mexico from the Mississippi River basin each year. Ofthis amount, 55% of the nitrogen released into the basin can beattributed to agricultural fertilizers with only a 3% contributionattributable to non-agricultural fertilizer application primarily onturf for lawns and recreational land (CAST, 1999).

Techniques to remotely measure crop status include the use of aspectroradiometer and other instruments (Bausch et al. 1994; Chappelleet al. 1992; Maas and Dunlap, 1989), aerial photography (Benton et al,1976), and satellite imagery.

The techniques listed above are not without their limitations. Forexample, early research by Resource21™ determined that during theoptimal fly over times between 10 a.m. and 11 a.m. for satelliteimaging, cloud cover had adverse affects on visibility. It was foundthat during the 10 am to 11 am time frame, fields in Colorado werevisible approximately 80% of the time while eastern Nebraska fields werevisible approximately 50% of the time. This trend in decreasedvisibility continued the farther east that data was collected. Also,spatial resolution for satellite imagery is poor (Landsat, 20 meter andpanchromatic, 10 meter). Similar problems plague aerial photographicmethods as well. While aerial imagery has better spatial resolution(typically less than 3 meters) than satellite imaging, partial cloudcover can shade sections of fields giving biased or incorrectreflectance measurements. Both techniques, however, suffer from the needfor extensive data processing (performed by third party providers athigh cost and long lead time) and geo-referencing issues. Even withspectroradiometric methods using sunlight as the ambient light source,cloud cover and time of day (8 a.m. to 8 p.m.) demands limit themainstream acceptance of the technology for addressing the nitrogen rateover-loading problem. What is needed is an on-the-go type sensor thatovercomes the time of day and fair-weather issues surrounding theaforementioned measurement techniques.

In certain crops or plant varieties, nutrient deficiencies constituteonly part of the management problem. In particular, the basic problem ofdetermining or monitoring plant status with respect to stress whether itstems from nutrient, water, pest, disease, or otherwise is of primaryconcern. For instance, turf stress determination is of major concern forthe turf manager. Earlier detection can protect the health of the grassbut also reduce the cost of restoring the badly damaged turf to goodhealth. Turf stress can be due to many causes such as water, pest,nutrient, heat, disease, and the like. By detecting changes in the turflandscape early, turf quality can be maintained and costly restorationoperations can be reduced or eliminated. On the other hand, being ableto control the degree of stress is important for some producers. Grapeproducers, for example, like to control the degree of water stress priorto harvesting in order to control disease and increase the sugar contentof the grape.

SUMMARY OF THE INVENTION

The new sensor of the present invention overcomes the time-of-day andfair weather limitations of passive technologies by incorporating itsown radiant source and by rejecting the influence of ambient light onthe measured canopy reflectance. Unlike passive sensor technology, thissensor will be able to operate under completely dark or full sunconditions. Additionally, the new sensor apparatus is an improvementboth in performance and cost over competing active-sensor technologiescommercially available.

As discussed above, the invention presented here will be advantageous ina number of commercial applications. For site-specific agriculturalapplications, the developed sensor would allow the producer to reducethe amount of nitrogen fertilizer applied to a crop or facilitatespoon-feeding the crop during the growing season, thus having thepotential for lowering production costs and enhancing environmentalquality. Also, by being able to determine the appropriate fertilizerneeds of the crop at any given location in the field, the producer canapply only the fertilizer needed to prevent yield loss or degradation ofproduct quality (i.e., protein content in wheat and barley or sugarcontent in sugar beets). Subsequently, decreased fertilizer rates willsubstantially lower nitrogen runoff and leaching losses, which willimprove the health of our watersheds, waterways, lakes, and oceans. Inaddition, data produced by the sensor may be used to produce relativeyield maps for forecasting crop production. As for turf grassapplications, the sensor technology would allow turf managers to mapchanges occurring on turf landscapes or for monitoring the status ofturf quality.

In accordance with the present invention, structures and methods areprovided for assessing plant status using the chlorophyll status changesand/or biomass properties of the plant remotely sensed, in the red-edgeportion of the vegetative reflectance spectrum (˜650 nm to ˜800 nm),thereby allowing selective monitoring or treatment of individual plants.

When incorporated into variable rate applicator and/or sprayers systems,the present invention significantly reduces the use of fertilizers byprecisely applying agricultural products to individual plants to betreated or eliminated. Moreover, the present invention is operable undera wide variety of conditions including cloudy conditions, brightsunlight, artificial illumination, or even total darkness. The advantageto the producer is that field operations do not have to be timed todaytime sunlight hours for operation.

All embodiments of the invention can be used in two primary ways. Thefirst method of use includes the application of the invention tohandheld instrumentation. Here the invention is utilized to measureplant canopies held in hand by a producer, turf manager, researcher, andthe like. The invention includes the use of GPS for geo-referencing datacollected by the invention. A second method of use includes applicationswhere the sensor is mounted a moving object such as a tractor, mower,center pivot/linear irrigator, or the like. Again, data may begeo-referenced using GPS for mapping and data layer (GPS maps, soilmaps, etc.) integration. Problem areas can be logged and reviewed laterby the producer or land manager for analysis and site managementdecisions.

An object of the invention is to provide a sensor for remotely sensingplant status using biophysical and biochemical properties of the plantthereby allowing selective monitoring, elimination, or treatment ofindividual plants.

This and other objects of the invention will be made apparent to thoseskilled in the art upon a review of this specification, the associateddrawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the effect of nitrogen rate onthe plant reflectance curve over the visible and near infrared portionof the spectrum.

FIG. 2 is a graphical representation of plant reflectance curves overthe visible and near infrared portion of the spectrum with the red-edgeportion of the spectrum emphasized.

FIG. 3 is a side and bottom view of a sensor of the present invention.

FIG. 4 is a functional block diagram of a preferred embodiment of thepresent invention.

FIG. 5 is a diagram of a circuit used to generate a light source of thepresent invention.

FIG. 6 shows diagrammatically a sensor based mapping system of thepresent invention.

FIG. 7 shows diagrammatically a sensor based variable-rate applicatorsystem of the present invention.

FIG. 8 illustrates preferred sensor-to-spray nozzle separation forcompensating for plant canopy periodicity and random leaf orientation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following contains a description for a sensor that remotely measuresplant canopy chlorophyll content independent of soil reflectance andambient illumination levels. The sensor can be used in stand-aloneinstrumentation configurations or in a network of sensors mounted to avehicle or moving apparatus for on-the-go remote sensing applications.The following description of the invention is meant to be illustrativeand not limiting. Other embodiments will be obvious in view of thisinvention.

The positive relationship between leaf greenness and crop nitrogen (N)status means it should be possible to determine crop N requirementsbased on reflectance data collected from the crop canopy (Walberg etal., 1982; Girardin et al., 1985; Hinzman et al., 1986; Dwyer et al.,1991) and leaves (McMurtrey et al., 1994), see FIG. 2. Plants withincreased levels of N typically have more chlorophyll (Inada, 1965;Rodolfo and Peregrina, 1962; Al-Abbas et al., 1974; Wolfe et al., 1988)and greater rates of photosynthesis (Sinclair and Horie, 1989). Hence,plants that appear a darker green are perceived to be healthier than Ndeficient plants and as such healthier plants reflectance less light inthe visible portion of the spectrum (400 to 700 nm) and reflect morelight in the near infrared (>700 nm), see FIG. 1. Chlorophyll in leavesabsorbs strongly in the blue 3 and red 4 regions of the spectrum (460 nmand 670 nm) and as the wavelengths increase past 670 nm the leaves beginto strongly reflect infrared light, see FIG. 2. The transition regionbetween the photosynthetic portion 1 (400 nm to 670 nm) and the biomassportion 2 (>780 nm) of a plant's reflectance spectrum is sometimesreferred to as the red-edge region 5. It has been reported in literaturethat the wavelength where the maxima of the derivative 6 for thered-edge band occurs is strongly correlated to changes in thechlorophyll status of a plant. Guyot and Baret (1988) developed analgebraic relationship expressing the wavelength of the red-edgeinflection point (REIP) 6, sometimes referred to as the red edgeposition (REP), using four reflectance bands spanning from 670 nm to 780nm. The usefulness of measuring red-edge reflectance spectra, andsubsequently determining the inflection point's wavelength position, isthat the chlorophyll status of the plant can be measured independentlyof soil background interference. That is, the chlorophyll status asdenoted by shifts in the red-edge inflection point is independent of theslope of the vegetative reflectance curve and has reduced sensitivity tosoil and biomass reflectance characteristics. Shifts in the value of theinflection point are directly related to the chlorophyll status (andwater content) of the plant with chlorophyll content being closelyrelated to nutrient status. Another useful red edge parameter is the redwell position 7 (RWP). This is the point on the vegetation reflectancecurve that represents the plants minimum reflectance, i.e., thewavelength of maximum chlorophyll absorption. This parameter, like theREEP, is also useful in determining changes in a plant's chlorophyllstatus.

There are two general embodiments of the invention that can be utilizedto measure red-edge reflectance. In the first embodiment of thisinvention, discrete monochromatic emitters providing coincident lightbeams; the beams are substantially in the red-edge portion of thevegetative reflectance spectrum (650 nm to 880 nm) and are sequenced onand off with respect to each other. The light source may be composedmost preferably from two or more emitter banks having differentwavelengths. The light beams illuminate a surface area on the plant'scanopy, which may include bare ground and desired plants. The reflectedlight signals are then detected by a single photodetector. In a secondembodiment, a chromatic or polychromatic emitter (a light source made upof multiple monochromatic emitters pulsed on/off in synchrony) and aspectrally sensitive detector array (e.g. four photodiodes fitted with10 nm bandwidth interference filters having center wavelengths of 670nm, 700 nm, 740nm and 780 nm). As in the previous embodiment, the lightbeam illuminates a surface area on the plant's canopy, which may includebare ground and desired plants. Each embodiment utilizes a controllerfor analyzing reflectance signals measured by the instruments and,assuming a plant is detected, responds by activating a device to takesome action with respect to the plant or stores the analyzed signal withcorresponding DGPS position in the controller's memory for lateranalysis. A number of actions may be taken by the controller. If theplant is a crop that is determined to be lacking in nutrient, thedesired action may be to apply fertilizer. Additionally, if the plantunder test is a turf landscape, such as found on golf courses andsporting fields, plant chlorophyll and/or biomass may be mapped andgeo-located using GPS for later, comparative analysis.

In an improvement on the first embodiment of this invention, three ormore light emitters (preferably four) provide selectively modulatedmonochromatic light beams of different wavelengths. The light beamswould preferably have the emission wavelengths of along the red-edgeportion of the plant's vegetative reflectance spectrum. The preferablewavelengths for the embodiment utilizing four monochromatic lightsources include the following wavelengths: 670 nm, 700 nm 740 nm and 780nm. For example, one could fabricate a light source composed of discreteLED light sources having wavelengths of 660 nm, 700 nm, 740 nm and 780nm. It will be apparent to one skilled in the art that other wavelengthsalong the red edge may be utilized in place of the four aforementionedwavelengths and that similar results will be obtained. Each of theselight sources would preferably have a spectral-line half-widths of lessthan 15 nm. Each monochromatic light source is modulated such that eachof the beams illumination time is non-overlapping with the other lightbeams or partially overlapping as occurs when the beams are staggered bya slight phase shift or modulated at harmonic frequencies. These lightbeams illuminate a small surface area on the ground, which, again, maybe bare ground, desired plants or undesired weeds. A single detectorsenses portions of the monochromatic light beams reflected by thesurface area and provides a quantitative signal indicative of thechlorophyll content of the plant. The signal produced can be integratedinto a controller and processed as in the previous embodiment in orderto determine plant chlorophyll content.

In both of the above embodiments, the red visible wave bands (660 nm to680 nm) and the long wave near infrared bands (760 nm to 880 nm) may beutilized to calculate classic biomass vegetative indexes such asnormalized difference vegetative index (NDVI), simple ratio index (SRI),etc... Additionally other unique vegetative indices, sensitive tochlorophyll can be formulated utilizing the wavebands along the rededge. For example, one could use a 730 nm LED and a 780 nm LED toilluminate the canopy in order to measure the a plant's chlorophyllcontent. The reflectance ratio of these two wavebands is proportional tothe chlorophyll status of the plant. Albeit, this ratio may be somewhatsensitive to soil background interference, it will produce good data forcanopies with LAI's greater than 2, that is, canopies that have morecomplete closure.

Prior art cited in U.S. Pat. No. 3,910,701 teaches the use of multipleLED wavelengths for plant status determination. And the use ofwavelength differentials (slopes) for comparative determination of plantstatus. This art, however, makes no distinction on the use the red-edgeportion of a plant's reflectance spectrum for qualitative chlorophyllassessment. Prior art cited in U.S. Pat. No. 5,789,741 make nodistinction with respect to chlorophyll content measurement but ratherrefer to changes in the slope of the vegetative reflectance spectrum asbeing indicative of the presence or absence of plant material ascompared with soil background reflectivity. The resultant measurementmade by the invention of '741 will be heavily influenced by soilbackground interference. Furthermore, this invention does notincorporate the spectral reflectances around the red-edge vegetativereflectance curve but rather detects slope changes as they deviate fromthe soil background line (but still heavily influenced by soilbackground interference); one slope calculated from 600 nm to 670 nm andthe other from 670 nm to 780 nm. As such, one trained in the art willnote that data produced by this method (U.S. Pat. No. 5,789,741) offerslittle benefit over biomass calculation methodologies via data producedby prior art referenced in U.S. Pat. Nos. 5,296,702, 5,389,781,5,585,626 and 6,596,996

While each of the two embodiments previously discussed have differentmodes of operation, the fundamental electronic instrumentation requiredto realize the two devices share many common features and in many waysare essentially the same. A discussion electro-optic elements requiredto realize each of the embodiments follows.

FIG. 3 shows a diagram of the sensor enclosure. The enclosurefacilitates the protection of the electronic circuitry while providingoptical emission and reception ports for the light source and the lightdetector components, respectively, of the sensor. Port 30 in FIG. 3 isthe emitter port of the sensor while port 31 is the detector port of thesensor. Port 30 and port 31 can facilitate various types of opticalcomponents to concentrate and collect optical energy. The type of opticsused by the sensor can include lens, mirrors, optical flats, filters,and diffusers. The type of optics selected for the emitter and detectoroptics depends on the application; that is, the required field of view,the height the sensor will be operated above the plant canopy, therequired cost of the sensor all may play a part in the design of thesensor's optical arrangement. The sensor can operate at a distance of 1foot and up to 10 s of feet from the plant canopy or surface of interestbut is not limited to this specific range. To those skilled in the artit should be readily apparent that fore optics on the emission side andthe detection side can take on many forms.

For example, a useful optically adaptation on the detector side of theoptical arrangement would be to encapsulate the detector optics (filtersand detectors). The outer optical surface would have a convex surfacespaced from the plane of the photodiode so as to create an afocal ornearly afocal optical arrangement. This preferred mode of constructionimproves the optical energy collection performance of the filter/diodecombination while sealing the optical path from dust and water vaporcondensation.

On the emission side of the sensor, there are a number of ways in whichto shape and direct the light beam emitting from the sensor body. Forinstance, if one wishes to generate a line pattern from the sensorslight source, preferably a bank of LEDs, one could place a cylindricallens in front of this light source spaced appropriately so as to image aline of illumination in the field of view of the detection optics.

Alternately, a circular or ellipsoidal area of irradiance can beproduced using only the encapsulation optics of an array of LEDs. Inthis instance, the beam pattern produced by the source is defined by thespatial irradiance distribution of each individual LED. No additionalcollimation or focusing optics is incorporated. Encapsulated LEDs can bepurchased commercially that have spatial distribution angles of 4degrees to almost 180 degrees. Most preferably, it is best to collimatethe light emitted form an LED in order to maintain a light beam withrelatively constant irradiance over distance. In this case the LED orLED array would be spaced an appropriate distance from a convex lens (orconcave mirror) to form an afocal or nearly afocal optical system. Theresulting optical system will produce a light beam that will becollimated along the optical axis of the light source resulting in areasof illumination with high radiance.

FIG. 4 shows a system diagram typical for the many embodiments of theinvention. The sensor is composed of optics to facilitate optical energycollimation and collection, a modulated light source 41 comprised of oneor many banks of polychromatic LEDs and/or monochromatic LEDs withassociated modulated driver and power control electronics 42, single ormultichannel photodetector array 43, high-speed preamplifier(s) withambient light cancellation 44, a phase sensitive signal conditioning 45and data acquisition circuitry 46, and a microcontrol unit (MCU) ordigital signal processor (DSP) 47 and an input/output interface 48 tocommunicate sensor data to an operator or controller. These systemelements will be discussed in the following.

The light source for the invention is most preferably composed of lightemitting diodes. LEDs are convenient light sources for this type ofinvention for a number of reasons. First, LEDs are available in a numberof colors useful for making plant biomass and pigment measurements. LEDsare readily available in colors spanning from deep violet (395 nm) tomid infrared (>4 um). Second, LEDs are extremely easy to use and can bemodulated to megahertz frequencies. Relatively simple electronic drivercircuits can be implemented and easily controlled by sensor controllerelectronics. Last, LEDs have long lifetimes and are rugged. The typicalLED will operate between 80,000 and 100,000 hours depending on thequiescent device power and operating temperature range.

LEDs are crystalline materials composed of various transition elementsand dopants that include gallium, arsenic, phosphorous, aluminum,nitrogen and indium. Common material chemistries for LEDs are GalliumArsenide (GaAs), Gallium Arsenide Phosphide (GaAsP), Gallium AluminumArsenide (GaAlAs), Indium Gallium Nitride (InGaN). Gallium nitride(GaN), Indium Gallium Aluminum Phosphide (InGaAlP), and GalliumPhosphide (GaP). Material chemistries that include GaN and InGaN aretypically utilized to produce LEDs that emit blue (400 nm) and green(570 nm) light. InGaAlP chemistries emit light in the green (560 nm) tored (680 nm) region of the spectrum while GaAs and GaAlAs emit light inthe red (660 nm) to near infrared (950 nm) region of the spectrum. LEDscan be purchased in encapsulated packages or in die form. Encapsulatedpackages have the benefit of providing mechanical robustness whilereducing Fresnel losses associated with a die/air interface.

LEDs are noncoherent light sources and their emission characteristicclassified as being mostly monochromatic or quasi-monochromatic, thatis, the frequencies composing the light are strongly peaked about acertain frequency. The spectral characteristic of an LED is defined byan emission band having a center wavelength (CWL) and a spectral-linehalf-width. The center wavelength defines the peak emission wavelengthof the LED and the spectral-line half-width defines the spectralbandwidth of the LED. Two other types of LED emitters can be classifiedas either having polychromatic or chromatic emission characteristics.Polychromatic LED's have a spectral signatures that are defined byhaving two or more distinct emission peaks. An example of an LED havingthis characteristic is the TLYH160 manufactured by Toshiba Corporation(Tokyo, Japan). This device emits simultaneously at 595 nm and 880 nm.Another example would include the True White LED manufacutred byAmerican Opto Plus (Pomona, Calif.). This particular device is an RGBLED that produces white light from the simultaneous emission of threedifferent LED structures (red, green and blue) on the same LED die. Asnoted above, LED's can also have chromatic emissions that produce abroad spectral emission signature. LED's of this type utilize a phosphorthat is applied over an LED die and then encapsulated with a transparentepoxy resin. A blue light LED is utilized to stimulate the phosphor toemit a white or colored broadband light. The emitted light has a broademission spectrum similar to that of fluorescent light. Both of theaforementioned LED types can be rapidly modulated to produce highlyeffective illumination signals for active light sensors.

In order to achieve good output stability with respect to thermal andaging effects, the LED sources should be adequately driven andmonitored. The output intensity of LEDs is very temperature dependent.Depending on the material type, an LEDs output can drift between 0.4 %/Cand 1 %/C. A decrease in output intensity, even if it is being monitoredand corrected via calculation, can result in diminished signal to noiseperformance of the measurement.

FIG. 5 shows schematically a circuit that provides active power controlfor the light source and an output intensity signal for monitoring andcalibration. Control voltage 50 sets the output power of light source51. Photodiode 52, an Infineon SFH203 (Munich, Germany), samples part ofthe output intensity of light source 51 and feeds this signal viaamplifier 53 to servo amplifier 54. Modulation of the output signal isperformed using transistor 55. Furthermore, the output of amplifier 53can be utilized to monitor the light source intensity for purposes ofcalibration and diagnostics. The performance of this circuit hasprovided output intensity control of approximately 0.05%/C over theoperating range of the invention. Many techniques have been discussed inliterature detailing methods on maintaining and stabilizing lightsources for photometric type measurements including the method presentedhere. As those skilled in the will note, there are numerous techniquesand methodologies for light source power monitor/stabilization forphotometric measurements discussed in engineering and scientificliterature.

The detectors used in the invention are most preferably siliconphotodiodes however other detector technologies such as GaAsP and thelike, may be utilized as well. Silicon detectors have a typicalphotosensitivity spanning from 200 nm (blue enhanced) to 1200 nm. Bandshaping of the detectors is performed using filtering materials such ascolored filter glass, interference filters or dichroic filters.Combinations of the aforementioned filter techniques can be combined inorder to band-shape the radiation impinging on the photodetectorsurface. For example, an interference filter can be used to select anarrow bandwidth of light. In this situation, one could choose to use a10 nm interference filters to select a band of interest along thered-edge portion of the vegetative reflectance spectrum. Utilizing anarray of photodetectors fitted with interference filters would providethe wavelength selection needed to realize the invention of embodimenttwo that utilizes a polychromatic source for illumination. In the caseof embodiment one that uses only a single photodetector, theincorporation of an long pass edge filter can be utilized to trim thephotodiode response by blocking short wave light and subsequentlyimproving the ambient light rejection of the associated preamplifierelectronics. As one trained in the art will see, there are numerous waysin which various optical filters can be utilized to shape and controlthe light impinging on a photodetector or photodetector array. A uniqueconfiguration petaining to embodiment two involves the use of lineardiode array detector and diffraction grating (or linear variable filter(LVF) technology). The diffraction grating (or LVF) separates incoming,modulated light in to many wavelengths. By configuring embodiment onewith a diffraction grating (or LVF)/linear array combination sensitiveto the red edge region of the vegetative reflectance curve, plantchlorophyll concentrations can be measured independent of soilbackground interference.

Referring once again to FIG. 4, both embodiments of the inventionutilize a phase sensitive detector subsystem (PSD) 45 andanalog-to-digital converter 46 (ADC) after each photodetector. The PSDs,sometimes referred to as lock-in amplifiers, are utilized by theinvention to extract and further amplify the very small signals detectedand amplified by the photodetector preamplifier(s). PSDs are often usedin applications where the signal to be measured is very small inamplitude and buried in noise. Detection is carried out synchronouslywith modulation of the light sources. Phase sensitive detection is oneof many types of band narrowing techniques that can be utilized tomeasure small signals. As will be apparent to those skilled in the art,other methods include the use of averaging techniques, discriminatorsand direct digital conversion/processing. With respect to direct digitalconversion/processing, the phase sensitive acquisition component can beperformed internally to a MCU or DSP by directly sampling the output ofthe photodiode amplifiers and performing the band pass and PSD functionsdigitally. By performing these operations in the digital domain, thetemperature drift of the phase detector, common to analog techniques,can be eliminated. The invention performs the synchronousmodulation/demodulation at a carrier frequency of 250 kHz. It should benoted that the operation of the invention is not limited to thisparticular modulation rate and can operate at other modulationfrequencies as well with as much effectiveness. Additionally, this ratecan be increased or decreased as dictated by the application. The MCU orDSP samples the output of a PSD 45 utilizing ADC 46. The resolution ofthe ADC is most preferably 12 bits. Each channel can sampled using adedicated ADC or one ADC can be utilized to sample all channels via amultiplexer.

Once the detected optical signals are amplified, demodulated andquantified, the MCU or DSP 47 can calculate chlorophyll content and/or avegetative relationship based on the reflectance values sensed.Calculations for plant chlorophyll status based multiple red-edgereflectance spectra can be performed a number ways. For the situationwhere the instrumentation has been designed to measure four or morereflectance values along the red edge, polynomial fitting may be used tofit the curve represented by the reflectance points. Subsequently, theresulting polynomial may be differentiated to find the red-edgeinflection point value. The resulting wavelength will be proportional torelative shifts in the chlorophyll status of the plant. When fourreflectance values are measured, the four reflectances having the centerwavelengths of 670 nm, 700 nm, 740 nm and 780 nm, a preferred method isthe four-point interpolation method. This method has the followingmathematical for $\rho_{i} = \frac{\rho_{1} + \rho_{4}}{2}$$\lambda_{i} = {\lambda_{2} + {\left( {\lambda_{3} - \lambda_{2}} \right) \cdot \frac{\rho_{i} - \rho_{2}}{\rho_{3} - \rho_{2}}}}$Where λ₁, λ₂, λ₃ and λ₄ are wavelengths 670 nm, 700 nm, 740 nm and 780nm, respectively, and ρ₁, ρ₂, ρ₃ and ρ₄ are reflectances at thecorresponding wavelengths, respectively. Additionally, another red edgeparameter, the red well position RWP, may be calculated using these samewavebands. The RWP interpolation has the following mathematical form$\lambda_{0} = {\lambda_{1} + {\left( {\lambda_{2} - \lambda_{1}} \right) \cdot \frac{\rho_{i} - \rho_{2}}{\rho_{3} - \rho_{2}}}}$The RWP represent the wavelength position of a plants minimumreflectance in the red, or rather the position of maximum chlorophyllabsorption. The RWP functions in a similar fashion as the REIP forpredicting relative changes in plan chlorophyll status.

Other mathematical techniques for determining the REIP and RWP includeLagrangian interpolation, inverted-Gaussian modeling, regressionmodeling, etc. . . . As will be apparent to one skilled in the art, thelist of the aforementioned methods is not exhaustive and other commonapproaches to determining the REIP and RWP wavelength positions may beformulated or found in literature.

In another useful red-edge sensor embodiment, a red polychromatic LED,such as the one utilized in U.S. patent application Ser. No. 10/703256,and a monochromatic LED, with an emission wavelength in the red edgeportion of a plant's vegetation reflectance spectra (680 nm to 760 nm),are utilized in a sensor that can distinguish between both plantnutrient and water stresses via the Canopy Chlorophyll Content Index(CCCI). The sensor utilizes two particular vegetation indexes. They area Normalized Difference Red-Edge (NDRE) index which has the followingmathematical form:${NDRE} = \frac{\rho_{3} - \rho_{2}}{\rho_{3} - \rho_{2}}$and a standard Normalized Difference Vegetation Index (NDVI) which hasthe form: ${NDVI} = \frac{\rho_{3} - \rho_{1}}{\rho_{3} - \rho_{1}}$Where ρ₁, ρ₂ and ρ₃ are reflectances at wavelengths 650 nm, 720 nm and880 nm. The NDVI as an estimate of percent plant cover and the NDRE asan indicator of plant chlorophyll content. The CCCI formula utilizesboth the NDVI and NDRE indexes to calculate the impact of water andnutrient on a crop or plant. The benefits of utilizing the polychromaticand monchromatic LED combination are many. First, the use of thepolychromatic LED reduces the number of LED banks from three to two.Second, because the number of LED banks have been reduced, the number ofLEDs in a particular bank can be increased which subsequently enhancingthe sensor' signal-to-noise performance. Third, because there are fewerbanks to modulate, the modulation rate can be higher for the LED banksthat are incorporated and subsequently enhancing the sensor'ssignal-to-noise performance. Last, easier temperature control andcompensation can be performed with fewer LED banks.

Data calculated by the sensor's processing component is communicated toan operator or system controller via input/output interface 48. In thecase of a handheld instrument, the I/O interface may take the form of akeypad and display. If the invention is incorporated into a sprayer ormapping system having several sensors networked together, the I/Ointerface will most preferably be a networkable serial port such a asRS485 port or CAN 2.0b port.

Applications of Use-Methods

FIG. 6 show a block diagram of the invention incorporated into a systemthat is used to map plant status. Elements of the system include sensorarray 60, sensor controller 61, and GPS 62.

The role of the sensor in this system is to measure the chlorophyllstatus and/or biomass properties of the plant being mapped. Dataproduced by the sensor are collected by the system controller forstorage and later analysis. Each sensor point is geo-referenced usingthe GPS connected the system controller. There are two primary ways inwhich mapping can be performed the system. First, the map collected bythe system can be all-inclusive, that is, every data point measured bythe sensor can be stored away in the controller's memory for laterretrieval and analysis. Second, the sensor/controller can be programmedwith a defined set of rules so as to distinguish poor performing regionsof a landscape from good or healthy regions and vice versa and storeonly the poor performing regions. This mode of operation saves storagespace in the controller and reduces the amount of data processing thathas to be performed. As an example, the mapping systems could be mountedto the mower machinery for a golf course. When the course personnelperform their weekly mowing operations, the mapping systems would scoutfor problem areas of the turf. For turf management operations, this modewould be most useful because regions of turf that are suffering fromstress (disease, water, nutrient, and so forth) or are beginning tosuffer. The mapping systems would flag affected areas for the turfmanager to scout out visually.

FIG. 7 show a block diagram of the invention incorporated into a systemthat is used for applying an agricultural product. Elements of thesystem include sensor array 70, sensor controller 71, GPS 72, fertilizercontroller 73, sprayer pumps/actuators 74 and ground speed sensor 75.

The agricultural product may be either in liquid or solid form and maybe, but not limited to, a nutrient, mineral, herbicide or fungicide or acombination of the aforementioned materials. The variable rate controlsystem can be mounted to a commercial sprayer or tractor mounted sprayersystem. GPS can be incorporated in the system when a map is required ofplant canopy characteristics for later analysis. In addition, to mappingplant characteristics, material dispensation rates can be mapped aswell. GPS is also required when applying fertilizer referenced to an Nsufficient reference strip. In this situation, a region of the field isgiven an N-rate that totally meets the needs of the crop to grow withoutloss of yield and apply a lower amount of pre-emergent fertilizer (onlythe amount to initially cause the crop to grow) to the remainder of thefield. At a time later in the growing season, the producer will apply asecond treatment to the remainder of the field using the sensor readingsfor the N sufficient region of the field. Readings from the Ninsufficient parts of the field will be compared with readings from theN sufficient regions of the field. The controller will use the sensormeasurements to calculate the appropriate rate of fertilizer to apply tothe N insufficient portion of the field in order to prevent yield loss.FIG. 8 shows an applicator example with the sensor stood-off from thespray nozzles. When designing variable rate application system, theobvious approach is to physically locate the sensor close or next to thesprayer nozzle. However, because of the random orientation of most plantcanopies the sensor should be separated from the sprayer nozzles by adistance D 80. This allows the sensing instrument to collect data on aportion of the crop, so as to average the spatial variability, beforeapplying an agricultural product. The separation distance D between thesensor and sprayer nozzles should most preferably be greater than 3feet. In operation, the variable rate system will collect data for Dfeet and apply an agricultural product over D feet while sensing thenext D separation distance. Another strength of a red-edge measurementsensor, as disclosed above, is that the measurement made by theinstrument is relatively invariant with respect to varying plantpopulation. This is critical for making N fertilizer recommendations onfields that have had crops planted utilizing variable rate seedingtechniques. With a biomass sensor, a seed rate map would have to beutilized in conjunction with the variable rate application algorithm inorder to compensate for changes in plant biomass resulting from theseeding operation.

The benefits of a system such as the one just described are botheconomic and environmental. By using less fertilizer and only applyingit where the crop needs it, the producer can lower his use of fertilizerand thus lower his production cost. Additionally, by using lessfertilizer and only applying it where the crop needs it, reduced run-offand leaching into our watershed occurs. Because the present inventionproduces its own source of light, the measurements that it makes is notinfluenced by ambient light conditions. Applicator equipment fitted withsensors of this type can be operated around the clock at night and underfull sun.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the embodiments givenwithout materially departing from the novel teachings and advantages ofthis invention. Accordingly, various modifications, adaptations, andcombinations or various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

1. Apparatus for assessing the status of plants in a plant canopy,comprising: (a) a light source having an emission spectrum substantiallyin the red-edge portion of the plant's reflectance spectrum forilluminating the plant canopy; (b) a detector for detecting portions ofthe emission spectrum reflected off of the plant canopy; and (c) aprocessor for determining the wavelength of the red edge inflectionpoint of the light collected by the detector.
 2. Apparatus as defined inclaim 1, further comprising a processor for determining the chlorophyllcontent of the plant from the red edge inflection point.
 3. Apparatus asdefined in claim 1, wherein the light source consists of a plurality ofmonochromatic light sources.
 4. Apparatus as defined in claim 1, whereinthe light source consists of one or more polychromatic light sources. 5.Apparatus as defined in claim 1, further comprising a mapping system forgenerating a map of the assessed plant status over a selected area. 6.Apparatus as defined in claim 1, further comprising an applicatorresponsive to the processor to apply a horticultural material to theplant canopy in response to the assessed plant status.
 7. Apparatus asdefined in claim 6, wherein the horticultural material is selected fromthe group consisting of fertilizer, herbicide, insecticide, fungicide,and combinations of such materials.